The present disclosure relates to a battery and a battery pack. In particular, the present disclosure relates to a battery and a battery pack using a laminate film of a metal layer and a resin layer.
In recent years, with rapid advancement in size and weight reduction of mobile information terminals, such as mobile phones, notebook-type personal computers, PDAs (Personal Digital Assistants), and the like, a high-capacity battery is increasingly demanded as a drive source. A nonaqueous electrolyte secondary battery which is represented by a lithium-ion secondary battery has a high energy density and a high capacity, so nonaqueous electrolyte secondary batteries have been widely used as a drive source for mobile information terminals, a drive source for power tools, and in-vehicle drive sources.
With the reduction in the size and weight of an electronic apparatus, there is demand for reduction in size and weight of electrical parts which are used in the electronic apparatus. For this reason, a nonaqueous electrolyte battery in which a bag-like body is used as a sealed vessel, and a nonaqueous electrolyte, a positive electrode, and a negative electrode are sealed in the bag-like body is being increasingly adopted as a power source.
The sealed vessel should have properties capable of preventing not only permeation of electrolyte and gas but also penetration of moisture from the outside. Therefore, as the material for the sealed vessel, a laminate film having a multilayer structure of resin film/metal layer/thermal adhesive resin (thermal adhesive layer) is used.
A nonaqueous electrolyte, a positive electrode, a negative electrode, and a separator arranged between the positive electrode and the negative electrode are sealed in the sealed vessel. Lead conductors with one end connected to the positive electrode and the negative electrode are arranged so as to extend from an opening of the sealed vessel to the outside of the sealed vessel, and the opening is sealed by fusion-bonding. In this way, the nonaqueous electrolyte battery can be made.
At a thermal adhesive portion (hereinafter, referred to as a seal portion) of the opening from which the lead conductors are extended, opposing thermal adhesive layers are bonded together. At a portion of the seal portion where a lead conductor is interposed between the thermal adhesive layers of the laminate film, the thermal adhesive layers of the laminate film are bonded to the lead conductor.
The seal portion should have not only excellent adhesion and sealing properties by fusion-bonding but also properties of not short-circuiting between the lead conductor and the metal layer disposed inside the thermal adhesive layer of the laminate film as a result of deformation during fusion-bonding.
For this reason, various contrivances have been made with respect to a layer disposed between the lead conductor and the metal layer of the laminate film. Further, an insulating coating layer is provided so as to coat the lead conductor, and various contrivances have been made with respect to the insulating coating layer.
Japanese Patent No. 3114719 discloses a technique in which a layer of maleic acid modified polyolefin having good adhesion with the lead conductor is provided as an insulating coating layer on the lead conductor and further a layer of cross-linked polyethylene having a gel percentage of 20% to 90% is provided outside the insulating coating layer.
JP-A-2001-297736 discloses a technique in which fusion-bonding is enabled at low temperature and the sealing effect is improved by interposing acid-modified linear low-density polyethylene between a lead conductor and a sealed vessel.
JP-A-2003-7269 discloses a technique in which a film made by laminating a high-fluidity polypropylene layer, which easily deforms due to heat and pressure applied during heat-sealing, and a low-fluidity polypropylene layer, which does not so easily deform is provided between a lead conductor and a sealed vessel. With this technique, short-circuit between a lead conductor and the metal layer of a laminate film can be prevented.
In recent years, a battery is increasingly used as an in-vehicle battery or the like in which large-current discharge will be made. In the case of large-current discharge, in order to reduce heat generated from the lead conductor at the time of discharge, it is necessary to increase the width of the lead conductor or the number of lead conductors. When the lead conductor has a large width, it is difficult to extend the lead conductors of the positive electrode and the negative electrode from the same side of the battery. For this reason, it is necessary to extend the lead conductor of the positive electrode and the lead conductor of the negative electrode from different sides of the battery.
However, according to the technique disclosed in Japanese Patent No. 3114719, the adhesion property varies depending on the degree of cross-linking of cross-linked polyethylene. For this reason, the technique disclosed in Japanese Patent No. 3114719 has a problem in that productivity is degraded because of control that should be made to secure adhesion by correctly controlling the degree of cross-linking.
The technique disclosed in JP-A-2001-297736 has a problem in that short-circuit is likely to occur due to softening or flowing of the interposed film at the time of fusion-bonding, and it is rather difficult to achieve fusion-bonding by controlling the application of pressure and heat so as to avoid the short-circuit.
The technique disclosed in JP-A-2003-7269 has a problem in that, if fusion-bonding is performed under a temperature condition exceeding the melting point of low-fluidity resin, short-circuit is likely to occur, and it is rather difficult to achieve good adhesion under such conditions as to avoid the short-circuit.
If the lead conductor of the positive electrode and the lead conductor of the negative electrode are extended from different sides, the area of the seal portion increases, which causes degradation of volume energy density. When the number of lead conductors increases, when the opening from which the lead conductor is extended is fusion-bonded, the lead conductors are likely to be in contact with each other, so leakage or short-circuit is likely to occur. For this reason, when the number of lead conductors increases, it is difficult to extend a plurality of lead conductors from the same side of the battery.
Thus, it is desirable to provide a battery and a battery pack which achieve ease of control when a seal portion is fusion-bonded, are capable of fusion-bonding the seal portion with good adhesion and sealing properties, and are capable of suppressing occurrence of short-circuit between a metal layer and a lead conductor when the seal portion is fusion-bonded. It is also desirable to provide a battery which is capable of suppressing short-circuit between lead conductors and has good energy density per volume.